For image data encoding methods, fixed-length encoding and variable-length encoding are known. A code length of compressed data in the fixed-length encoding is fixed. A code length of compressed data in the variable-length encoding is not fixed. For example, it is known that in MPEG-2 which is an encoding scheme, the quantization parameter can be set in the range from 0 to 31, and 5-bit fixed-length encoding is performed. In addition, an encoding scheme such as H.264 is known to adopt variable-length encoding.
When variable-length encoding is performed, encoded data with a higher compression ratio compared to fixed-length encoding can be obtained. However, it is known that it takes more time to decode encoded data obtained by variable-length encoding than to decode encoded data obtained by fixed-length encoding. On the other hand, encoded data obtained by fixed-length encoding has a shorter processing time required upon decoding compared to variable-length encoding, but is known to have low compression ratio efficiency.
In addition, there is disclosed a technique for dividing encoded data having been subjected to variable-length encoding into fixed-length packets with a high level of importance and fixed-length packets with a low level of importance, and retransmitting the packets with a high level of importance upon transmission failure.
However, conventionally, it is difficult to perform high-speed decoding of encoded data having been subjected to variable-length encoding.